Method of automatically adjusting welding conditions for an arc welding robot

ABSTRACT

A method, such that, using known data, a relationship between a target welding current and a welding current command to be instructed to a welding machine to achieve the target welding current is estimated by a linear formula. Then, a welding current command corresponding to a welding current to be achieved is obtained according to the linear formula, and is actually supplied to the welding machine to carry out a welding operation. An actual welding current is fed back from the welding machine, and this actual welding current and the welding current command associated therewith are stored as one set of data. This process is repeated using different welding current command values, and the resulting sets of data each including the combination of a corresponding welding current command and actual welding current are stored. Based on a plurality of sets of data thus obtained, a linear relational formula, representing the relationship between the actual welding current (target welding current) and the current command, is derived for each of a plurality of welding current ranges.

TECHNICAL FIELD

The present invention relates to a method of automatically adjustingwelding conditions according to which arc welding is carried out by anarc welding robot.

BACKGROUND ART

As shown in FIG. 7, a robot control device 1 outputs a welding voltagecommand Vc and a welding current command Ic to a welding machine 2.Based on these commands Vc and Ic, a controller 3 of the welding machine2 operates the welding machine 2 to perform a welding operation.

The feed speed of a wire feeder 4 is determined by a welding current Ir,which is the output current of the welding machine 2. Therefore, controlof the feed speed of the wire feeder 4 is performed based on the currentcommand Ic supplied to the welding machine. In arc welding robots,welding conditions for the welding machine are controlled by means of ananalog voltage. Therefore, in practice, the current command Ic is givento the controller 3 of the welding machine 2 in the form of a voltage(Vic) corresponding to the command Ic.

Generally, there is no linear relationship between the wire feed speedand actual welding current. In other words, the current command Ic (Vic)and the actual welding current IF have a relationship as indicated bythe curve in FIG. 5, and not a linear relationship.

Conventionally, therefore, a current command Ic-actual current IF curve,which represents the relationship as shown in FIG. 5, is divided into aplurality of regions, then the curve segment in each subdivided regionis approximated by a linear formula, and the actual welding current Ir(i.e., the feed speed of the wire feeder) for the command weldingcurrent Ic is obtained by using the approximate formula. That is, alinear conversion formula for deriving the command current value Ic froma target welding current is prepared for each of the weldingcurrent-based regions.

More specifically, a database is constructed beforehand such that itstores welding voltage and current data as welding conditions to be usedin a welding environment including the material, diameter, etc., of awelding wire to be used, as well as a linear conversion formula uniquelyassigned to the welding current-based region for calculating a currentcommand value to be instructed to the welding machine to achieve atarget welding current. The linear conversion formula is set and storedin such a manner that it is finely adjusted in accordance with the shapeof a workpiece to be welded, the aiming angle of a torch, etc. Duringwelding, the command welding voltage and the command welding current(wire feed speed command) are supplied to the welding machine inaccordance with the data stored in the database.

In a welding operation utilizing such a database as described above,nearly satisfactory welding control is achieved as long as the weldingconditions do not greatly deviate from the initial set values. However,in cases where the welding conditions change greatly, for example, wherea different workpiece is to be welded, or the welding speed, i.e., therobot moving speed, is changed, the linear conversion formula used tillthen to convert the target welding current to a command current value isno longer applicable to the new welding conditions. Accordingly, theoperator must carry out trial welding a plurality of times according tothe new welding conditions, to obtain a new conversion formula. Thiswork causes an inconvenience to the operator and consumes labor.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a method ofautomatically adjusting welding conditions for an arc welding robot,wherein, even when the welding conditions change, a linear conversionformula for obtaining a command current value from a target weldingcurrent can be automatically modified according to new weldingconditions, by means of feedback values from a welding machine.

To achieve the above object, the present invention comprises the stepsof: estimating a relationship between a target welding current and awelding current command to be instructed to a welding machine to achievethe target welding current, by means of a linear formula; obtaining awelding current command value corresponding to a welding current valueto be achieved, by using the linear formula, and actually supplying thewelding current command value to the welding machine to carry out awelding operation; detecting an actual welding current of the weldingmachine during the welding operation, and storing the welding currentcommand value supplied during the detection and the actual weldingcurrent value as one set of data; supplying another welding currentcommand value different from the first-mentioned welding current commandvalue to the same welding machine to carry out a welding operation, andstoring the welding current command value and an actual welding currentvalue as another set of data; and obtaining at least three sets of dataeach including a combination of the welding current command and theactual welding current, and deriving a linear relational formularepresenting the relationship between the target welding current and thecurrent command based on the at least three sets of data.

Preferably, a welding current to be supplied to the welding machine isinstructed by means of a voltage corresponding thereto, and the linearformula is determined based on voltage values corresponding to maximumand minimum welding currents that can be instructed with respect to thewelding machine, and estimated welding current values respectivelycorresponding to the voltage values.

According to another aspect, the present invention comprises the stepsof: estimating a formula representing a relationship between an actualwelding current and a welding current command to be instructed to awelding machine to achieve the actual welding current; setting aplurality of target welding currents falling within one of subdividedwelding current ranges, and obtaining welding current commands forachieving the target welding currents, respectively, by using theformula; actually supplying each of the obtained welding currentcommands to the welding machine to carry out a welding operation, andfeeding an actual welding current falling within the one of thesubdivided welding current ranges back to a control device associatedwith an arc welding robot when desirable welding operation is performed;and obtaining, by the control device, a linear formula approximating therelationship between the target welding current falling within the oneof the subdivided welding current ranges and the welding current commandto be instructed to the welding machine to achieve the target weldingcurrent as actual welding current, based on a plurality of combinationseach including the fed back actual welding current and the commandcurrent supplied when the actual welding current is obtained.

Preferably, the welding current command is supplied as an analog voltageto the welding machine, and the formula estimated as above is a linearformula derived based on maximum and minimum voltage values ofpredetermined welding current commands that can be instructed withrespect to the welding machine, and maximum and minimum welding currentvalues estimated with respect to the maximum and minimum voltage values,respectively.

Still preferably, the welding current command is supplied as an analogvoltage to the welding machine, the actual welding current of thewelding machine is fed back as an analog voltage to the control deviceassociated with the arc welding robot and converted to an actual weldingcurrent value in the control device, and in the step of obtaining thelinear formula, the linear formula is derived by a least squares method,based on at least three fed back actual welding current values fallingwithin an identical welding current range and voltage values of thecurrent commands supplied when the respective three actual weldingcurrent values are obtained.

As described above, according to the present invention, a linearconversion formula for obtaining a command current value from a targetwelding current can be automatically derived by merely giving weldingconditions such as a welding current command to the welding machine tocause the machine to carry out trial welding, thus eliminatingcomplicated work to be done by the operator, such as adjustment andresetting of the conversion formula. That is, the number of times thetrial welding must be executed is reduced, and the operator's laborassociated with the resetting can be greatly reduced. Further, since thelinear conversion formulas derived during welding work in variouswelding environments can be stored up in the form of a database, theycan be referred to later when commands for welding work are created.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of a welding condition adjusting process accordingto an embodiment of the present invention;

FIG. 2 is a flowchart of a sampling process according to the embodiment;

FIG. 3 is a diagram illustrating how the relationship between a targetwelding current and a command value (welding current command voltage)for achieving the target welding current is estimated based on a linearformula;

FIG. 4 is a block diagram of a control system of an arc welding robotfop carrying out the embodiment of the present invention;

FIG. 5 is a diagram illustrating the relationship between an actualwelding current and an actual command value (welding current commandvoltage) for achieving the actual welding current;

FIG. 6 is a diagram illustrating how a linear formula indicating therelationship between target welding current and welding current commandvoltage is derived from data representing the relationship between anactual welding current and an actually instructed welding currentcommand voltage; and

FIG. 7 is a block diagram of a control system of a conventional arcwelding robot.

BEST MODE OF CARRYING OUT THE INVENTION

To obtain target welding voltage and current, a welding voltage commandand a welding current command are given to a welding machine as weldingconditions. In practice, the welding current command is supplied to thewelding machine in the form of a voltage (hereinafter referred to as the"welding current command voltage") corresponding to the current command.This is because all welding conditions for an arc welding robot arecontrolled by means of analog voltage.

The relationship between an actual welding current Ir and weldingcurrent command voltage Vic given to the welding machine from a robotcontrol device is nonlinear, as shown in FIG. 5. Therefore, the weldingcurrent command voltage Vic fop achieving a target welding current(actual welding current Ir) cannot be obtained by applying a singlelinear formula to the target welding current.

If, however, a plurality of magnitude-based ranges is set for thewelding current Ir to be outputted from the welding machine, then therelationship between the welding current Ir and the current commandvoltage Vic in each such range can be approximated by a single linearrelationship. Accordingly, for a target welding current Io fallingwithin a certain range, the relationship between the target weldingcurrent Io and a current command voltage Vic for achieving the targetwelding current Io (that is, as actual welding current Ir) can beexpressed as a linear formula as follows:

    Vic=a·Io+b                                        (1)

In this equation, a and b are coefficients applied to the specific rangewithin which the welding current Io falls, and generally need to beadjusted in accordance with the aiming angle of a torch, the shape of aworkpiece to be welded, etc. Where the target welding current Io fallswithin another range, the coefficients a and b in equation (1) haverespective different values.

Actual welding voltage Vr and actual welding current value Ir areoutputted from the welding machine. In practice, the actual weldingcurrent Ir is not fed back as it is, but is fed back to the robotcontrol device after being converted to a corresponding voltage valueVif, as shown in FIG. 4. To convert the voltage value Vif to actualwelding current value Ir, equation (2) below is used:

    Ir=c·Vif+d                                        (2)

Coefficients c and d in equation (2) are values specific to the weldingmachine and are uniquely determined when the welding machine to be usedis specified.

According to the present invention, the welding machine is operated tocarry out trial welding, the actual welding current Ir is calculatedfrom the voltage value Vif corresponding to the actual welding currentvalue Ir fed back from the operating machine according to theaforementioned equation (2), the relationship between the actual weldingcurrent Ir and the welding current command voltage Vic instructed whenthis actual welding current Ir is outputted is derived for each of thewelding current (Ir)-based ranges, to thereby obtain a linear relationalformula representing the relationship between the target welding currentIo and the welding current command voltage Vic for achieving the targetwelding current Io as actual welding current Ir, for each of the weldingcurrent ranges. In other words, the robot control device automaticallydetermines the coefficients a and b in the aforementioned equation (1)for each of the welding current ranges, by carrying out trial weldingusing the welding machine, and employing the voltage value Vif fed backfrom the welding machine and corresponding to the actual welding currentIr.

FIG. 4 is a block diagram of a control system of an arc welding robot towhich the present invention is applied. A robot control device 10includes a processor 11, which is connected via a bus 19 to a ROM 12 forstoring control programs, a RAM 13 for temporarily storing a weldingprogram specifying a travel path of a robot, welding commands, etc., andvarious data, a teaching control panel 14 for teaching a program to therobot and permitting entry of various commands, a control panel 15, anaxis controller 16 for controlling individual robot axes, and aninterface 17. The axis controller 16 is connected to a servo amplifier18 for driving servomotors associated with the respective axes of arobot body 20.

A welding machine 30 is connected to the robot control 10 device via theinterface 17. A welding voltage command Vc and a welding current commandvoltage Vic, each in the form of an analog voltage, are output from therobot control device 10 to the welding machine 30. On the other hand, awelding voltage feedback value Vf and a voltage Vif corresponding to adetected welding current, each in the form of an analog voltage, are fedback from the welding machine 30 to the robot control device 10.Further, other control signals are transferred between the robot controldevice 10 and the welding machine 30. The welding machine 30 has awelding torch attached to a wrist provided at a distal end of an arm ofthe robot body 20, and the torch moves along a taught path when therobot is operated. The interface 17 includes a D/A converter forconverting digital signals to analog signals, and an A/D converter forconverting analog signals to digital signals.

The arrangement of the arc welding robot described above is identical tothat of a conventional arc welding robot; therefore, a detaileddescription thereof is omitted.

An automatic adjusting method for welding conditions, according to anembodiment of the present invention, will be now described withreference to FIGS. 1 and 2. FIGS. 1 and 2 are flowcharts of processesexecuted when the robot control device 10 is set in an automatic weldingcondition adjusting mode.

First, the robot control device 10 is set in the automatic weldingcondition adjusting mode, and a voltage Vmax corresponding to a maximumwelding current command that can be instructed to the welding machine30, a voltage Vmin corresponding to a minimum welding current command,and estimated welding current values Imax and Imin that will flow whenthe maximum voltage Vmax and the minimum voltage Vmin, respectively, arespecified as welding current commands, are inputted to the controldevice 10. These values, i.e., the maximum and minimum voltages Vmax andVmin, and the estimated maximum and minimum currents Imax and Imin, areusually obtained in advance for each welding machine by experiments andthus are known. Also, the aforementioned formula (2) and the value of ε,mentioned later, are entered (Step S1).

On reading the input data, the processor 11 of the robot control device10 determines the relationship between an estimated welding current Ipand a welding current command voltage Vp in the form of a straight lineconnecting points (Imax, Vmax) and (Imin, Vmin) in a rectangular Ip-Vpcoordinate system, as shown in FIG. 3, on the basis of the read data(Step S2).

Provided the straight line in FIG. 3 is given by the following equation:

    Vp=e·Ip+f                                         (3)

coefficients e and f can be derived as follows:

    e=(Vmax-Vmin)/(Imax-Imin)

    f=Vmin-e·Imin

The straight line in the rectangular coordinate system of FIG. 3,indicated along a horizontal axis Ip (estimated welding current) and avertical axis Vp (welding current command voltage), is provisionally setbased on the known data as a substitute for the curve in the rectangularcoordinate system of FIG. 5, indicated along a horizontal axis Ir(actual welding current) and a vertical axis Vic (welding currentcommand voltage), since the curve is not known at this stage.

Subsequently, a welding voltage command Vc and a target welding currentIo1, as welding conditions for trial welding by means of the weldingmachine 30, are input from the control panel 15 to the robot controldevice 10. The processor 11 of the robot control device 10 substitutesthe input value Io1 for the variable Ip in equation (3), to obtain awelding current command voltage Vp=Vic1 corresponding to an estimatedwelding current Ip=Ip1 (see FIG. 3). The welding current command voltageVic1 and the welding voltage command Vc are then actually supplied tothe welding machine 30 (Step S3). Thus, the processor 11 actuates therobot in accordance with a trial welding program taught thereto, and thewelding machine 30 carries out welding operation in accordance with thewelding voltage command Vc and welding current command voltage Vci1supplied thereto (Step S4).

If no arc is generated when the welding machine is operated according tothe thus-instructed welding conditions, an alarm is supplied to therobot control device (Step S5). On receiving the alarm signal, the robotcontrol device 10 displays a message, etc., requesting modification ofthe welding conditions, at the display device of the teaching controlpanel 14 or the like (omitted from FIG. 1), and waits for the entry ofnew welding conditions. When new conditions are entered, the flowreturns to Step S3 and welding operation is carried out according to thenew conditions.

If an arc is properly generated as a result of trial welding operationby the welding machine 30 according to the instructed conditions, andthus no alarm is generated, a sampling process shown in FIG. 2 isexecuted (Step S6).

In the sampling process, a voltage Vif=Vif1 corresponding to the weldingcurrent fed back from the welding machine 30, which is carrying outtrial welding, is read (Step T1), and an actual welding current Ir1 isobtained based on the voltage Vif1, by using the preset equation (2) forconverting the voltage (corresponding to the welding current) to thewelding current (Step T2). The welding current command voltage Vic1actually instructed to the welding machine 30 and the obtained actualwelding current Ir1 are stored in the RAM as one set of data (Vic1, Ir1)(Step T3; see FIG. 5), and the flow returns to the main routine of FIG.1.

Then, the target welding current is set to a value Io2 which is greaterthan the value Io1 used in Step S6 by a predetermined amount. In thisembodiment, Io2 is defined as Io2=Io1·(1+ε), where ε is a preset valueand about 0.1. After determining the target welding current Io2 in thismanner, the processor 11 of the robot control device 10 substitutes thevalue Io2 for the variable Ip in equation (3), to obtain a weldingcurrent command voltage Vp=Vic2 for achieving the target welding currentIo2 (estimated welding current Ip2). The voltage command Vc and thewelding current command voltage Vic2 are actually supplied to thewelding machine 30, which then carries out a welding operation accordingto the thus-set welding conditions (Step S7).

While welding is performed by the welding machine 30, the processor 11of the robot control device 10 executes the aforementioned samplingprocess shown in FIG. 2 (Step S8). That is, a voltage Vif2 correspondingto the welding current fed back from the welding machine 30 is read, anactual welding current Ir2 is obtained according to the preset equation(2) for converting the voltage (corresponding to the welding current) toa welding current, the welding current command voltage Vic2 actuallyinstructed to the welding machine 30 and the obtained actual weldingcurrent Ir2 are stored in the RAM as one set of data (Vic2, Ir2) (StepsT1-T3), and the flow returns to the main routine of FIG. 1.

Subsequently, the target welding current is set to a value Io3 which issmaller than the value Io1 used in Step S6 by a predetermined amount. Inthis embodiment, Io3 is defined as Io3=Io1 (1-ε). After determining thetarget welding current Io3 in this manner, the processor 11 of the robotcontrol device 10 calculates a welding current command voltage Vp=Vic3for achieving the target welding current Io3, according to equation (3)in the same manner as described above, and supplies the welding currentcommand voltage Vic3, along with the voltage command Vc, to the weldingmachine 30, to actually operate the welding machine 30 according to thethus-set welding conditions (Step S9). Then, a voltage Vif3corresponding to the welding current fed back from the welding machine30 is read, an actual welding current Ir3 is obtained based on thevoltage Vif3 in the same manner as described above, the welding currentcommand voltage Vic3 actually instructed to the welding machine 30 andthe obtained actual welding current Ir3 are stored in the RAM as one setof data (Vic3, Ir3) (Steps T1-T3), and the flow returns to the mainroutine of FIG. 1.

Based on the thus-obtained three sets of data each including thecombination of welding current command voltage Vic and actual weldingcurrent Ir, a linear equation (i.e., the coefficients a and b inequation (1)), by which the target welding current Io is converted tothe welding current command voltage Vic for achieving the target weldingcurrent Io as actual welding current (Io=Ir), is obtained by the leastsquares method, as shown in FIG. 6. The obtained linear equation isstored, and the automatic welding condition adjusting process for onewelding current-based range is ended.

We claim:
 1. A method of automatically adjusting welding conditions foran arc welding robot, comprising the steps of:a) estimating arelationship between a target welding current and a welding currentcommand to be instructed to a welding machine to achieve the targetwelding current, by means of a linear formula; b) obtaining a weldingcurrent command value corresponding to a welding current value to beachieved, by using the linear formula, and actually supplying the firstwelding current command value to the welding machine to carry out awelding operation; c) detecting an actual welding current of the weldingmachine during the welding operation, and storing the first weldingcurrent command value supplied during the detection and the actualwelding current value as one set of data; d) supplying another weldingcurrent command value different from the welding current command valueobtained in said step b) to the welding machine to carry out a secondwelding operation, and storing another welding current command value andanother actual welding current value as another set of data; obtainingat least three sets of data, each including a combination of the weldingcurrent command and the actual welding current, and deriving a newlinear relational formula representing a relationship between the targetwelding current and the current command based on said at least threesets of data; using the new linear relational formula to determine a newwelding current command for a new target welding current.
 2. The methodaccording to claim 1, wherein the actual welding current to be suppliedto the welding machine is instructed by means of a voltage correspondingthereto, and the linear formula is determined based on voltage valuescorresponding to maximum and minimum welding currents that can beinstructed to the welding machine, and estimated welding current valuesrespectively corresponding to said voltage values.
 3. A method ofautomatically adjusting welding conditions for an arc welding robot,comprising the steps of:a) estimating a formula representing arelationship between an actual welding current and a welding currentcommand to be instructed to a welding machine to achieve the actualwelding current; b) setting a plurality of target welding currentsfalling within one of subdivided welding current ranges, and obtainingwelding current commands for achieving the target welding currents,respectively, by using the formula; c) actually supplying each of theobtained welding current commands to the welding machine to carry out awelding operation, and feeding the actual welding current falling withinsaid one of the subdivided welding current ranges back to a controldevice associated with the arc welding robot when a desirable weldingoperation is performed; d) obtaining, by the control device, a linearformula approximating the relationship between the target weldingcurrent falling within said one of the subdivided welding current rangesand the welding current command to be instructed to the welding machineto achieve the target welding current as the actual welding current,based on a plurality of combinations each including the fed back actualwelding current and the command current supplied when the actual weldingcurrent is obtained; and supplying the welding current commanddetermined in said step d) to the welding machine to achieve the targetwelding current falling within said one of the subdivided weldingcurrent ranges.
 4. The method according to claim 3, wherein each weldingcurrent command is supplied as an analog voltage to the welding machine,and the formula estimated in said step a) is a linear formula derivedbased on maximum and minimum voltage values of predetermined weldingcurrent commands that can be instructed to the welding machine, andmaximum and minimum welding current values estimated to result from saidmaximum and minimum voltage values, respectively.
 5. The methodaccording to claim 3, wherein each welding current command is suppliedas an analog voltage to the welding machine, each actual welding currentof the welding machine is fed back as an analog voltage to the controldevice associated with the arc welding robot and converted to an actualwelding current value in the control device, and in said step d), thelinear formula is derived by a least squared method, based on at leastthree fed back actual welding current values falling within an identicalwelding current range and voltage values of the current commandssupplied in response to the respective three actual welding currentvalues.
 6. The method according to claim 4, wherein each welding currentcommand is supplied as an analog voltage to the welding machine, eachactual welding current of the welding machine is fed back as an analogvoltage to the control device associated with the arc welding robot andconverted to an actual welding current value in the control device, andin said step d), the linear formula is derived by a least squaredmethod, based on at least three fed back actual welding current valuesfalling within an identical welding current range and voltage values ofthe current commands supplied in response to the respective three actualwelding current values.
 7. A method of determining a linear relationalformula between a target welding current and a welding current commandfor an arc welding robot, said method comprising the steps of:a)estimating a relationship between a target welding current and a weldingcurrent command to be instructed to a welding machine to achieve thetarget welding current, by means of a linear formula; b) obtaining awelding current command value corresponding to a welding current valueto be achieved, by using the linear formula, and actually supplying thefirst welding current command value to the welding machine to carry outa welding operation; c) detecting an actual welding current of thewelding machine during the welding operation, and storing the firstwelding current command value supplied during the detection and theactual welding current value as one set of data; d) supplying anotherwelding current command value different from the welding current commandvalue obtained in said step b) to the welding machine to carry out asecond welding operation, and storing another welding current commandvalue and another actual welding current value as another set of data;and obtaining at least three sets of data, each including a combinationof the welding current command and the actual welding current, andderiving a new linear relational formula representing a relationshipbetween the target welding current and the current command based on saidat least three sets of data.
 8. The method according to claim 7, whereinthe actual welding current to be supplied to the welding machine isinstructed by means of a voltage corresponding thereto, and the linearformula is determined based on voltage values corresponding to maximumand minimum welding currents that can be instructed to the weldingmachine, and estimated welding current values respectively correspondingto said voltage values.
 9. A method of determining a linear relationalformula between a target welding current and a welding current commandfor an arc welding robot, said method comprising the steps of:a)estimating a formula representing a relationship between an actualwelding current and a welding current command to be instructed to awelding machine to achieve the actual welding current; b) setting aplurality of target welding currents falling within one of subdividedwelding current ranges, and obtaining welding current commands forachieving the target welding currents, respectively, by using theformula; c) actually supplying each of the obtained welding currentcommands to the welding machine to carry out a welding operation, andfeeding the actual welding current falling within said one of thesubdivided welding current ranges back to a control device associatedwith the arc welding robot when a desirable welding operation isperformed; and d) obtaining, by the control device, a linear formulaapproximating the relationship between the target welding currentfalling within said one of the subdivided welding current ranges and thewelding current command to be instructed to the welding machine toachieve the target welding current as the actual welding current, basedon a plurality of combinations each including the fed back actualwelding current and the command current supplied when the actual weldingcurrent is obtained.
 10. The method according to claim 9, wherein eachwelding current command is supplied as an analog voltage to the weldingmachine, and the formula estimated in said step a) is a linear formuladerived based on maximum and minimum voltage values of predeterminedwelding current commands that can be instructed to the welding machine,and maximum and minimum welding current values estimated to result fromsaid maximum and minimum voltage values, respectively.